Selling an Apple replacement (iMessage etc)

I have heard of problems with selling iPhones and the phone still being linked to your account for iMessage.

If I do nothing to the Apple Replacement I should be fine right?

Also the phone should be unlocked as my last one was, at the moment I have no way of checking this (no spare sims) but since now iPhones are set up in a different way on the first start up, is there a way to be sure and then perhaps reset the phone so that the buyer gets the first time start up for settings etc?

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